High average current effects in energy recovery linacs

Energy recovery linacs (ERLs) can be used as high efficiency driver accelerators to free electron lasers, synchrotron radiation light sources and colliders. Energy recovery has been successfully demonstrated up to 5 mA of average current in the Jefferson Lab 1.7 kW IR FEL. Future designs call for much higher average currents of order 100 mA. A number of phenomena can potentially limit the performance of ERLs at these high currents. We review multibunch, multipass instabilities and present experimental data on transverse beam breakup and HOM power dissipation obtained at the Jefferson Lab IR FEL. We compare measurements with analytical calculations and simulations.